Method for providing a bi-directionally wound cell stack for enhanced battery performance

ABSTRACT

An electrochemical cell comprising an electrode assembly in which overlayed electrodes are wound together in a bi-directional fashion yielding a high energy density cell stack with low internal impedance is described. The overlayed electrodes are such that either a single cathode is paired with two anodes or a single anode is paired with two cathodes prior to winding of the cell stack assembly. For example, the electrode assembly is formed by overlapping the two anode electrodes on opposite sides of the cathode electrode across a midportion and then winding the electrode strips bi-directionally about the midportion.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is a continuation-in-part application ofSer. No. 09/262,245, filed Mar. 4, 1999.

FIELD OF INVENTION

[0002] The present invention generally relates to the art ofelectrochemical energy, and more particularly, to an electrode assembly,electrochemical cells in which the electrode assembly is used, and amethod for making the electrode assembly.

BACKGROUND OF THE INVENTION

[0003] Batteries or electrochemical cells are typically volumetricallyconstrained systems that cannot exceed the available volume of thebattery case. The size and resulting volume of the battery case aredictated by the space requirements available for the particularapplication. The components that make up a battery, namely, the cathodeelectrode, the anode electrode, the separator, the current collectors,and the electrolyte all have to fit into the limited space defined bythe battery case. Therefore, the arrangement of the components impactson the amount of active electrode material that can be fit into the caseand the ease of manufacturing the unit.

[0004] Some typical electrode assemblies include the “Z” foldedelectrode assembly that is disclosed in U.S. Pat. No. 3,663,721 toBlondel et al. In the “Z” folded electrode, a unitary and continuouslithium anode is folded back and forth in a zigzag fashion. The lengthof the individual folds determines the width of the electrode assembly.Individual cathode plates are positioned between pairs of the pleatedanode electrode and electrically connected to one another. The designhas some drawbacks, including the requirement that separate cathodeplates be inserted between each pair of adjacent layers of anodeelectrode and the requirement that electrical connections be madebetween all of the inserted cathode plates. This arrangement increasesthe time and costs associated with manufacturing.

[0005] Another typical electrode assembly configuration is the “jellyroll” design in which the anode electrode, the cathode electrode, andthe separator are overlaid with respect to each other and coiled up.Such an electrode configuration is desirable because the continuousanode and cathode electrodes require a minimal number of mechanicalconnections to their respective terminal leads, and the jelly rollassembly is generally recognized as preferred for high discharge andcurrent pulse applications. However, in some applications, acylindrically shaped electrode assembly is not desired because of otherfactors, such as the shape of the battery case.

[0006] U.S. Pat. No. 4,761,352 to Bakos et al. discloses yet anotherelectrode assembly design comprising an accordion folded electrodeassembly with unitary members for both the anode and cathode strips. Thecathode strip is approximately half the length of the anode strip, andthe anode strip is folded over the cathode strip to “sandwich” thecathode between two layers of the anode. The resulting form is thenmanually folded in an alternating series of “V” folds (best shown inFIG. 4 of the patent). However, that design provides some undesirablegaps which reduce the volumetric density of the electrochemically activematerials.

[0007] What is needed is an improved multi-layer, folded electrodeassembly design for high energy devices that includes many of thedesirable features of the jelly roll design, such as unitary anode andcathode electrodes.

SUMMARY OF THE INVENTION

[0008] The present invention fills the above-described need by providingan electrochemical cell comprising an electrode assembly in which theelectrodes are wound together in a bi-directional fashion, yielding ahigh energy density cell with low internal impedance. The anode andcathode electrodes are arranged in the cell in such a fashion thatprovides efficient utilization of the active components. The resultantwound assembly is configured such that it can be conveniently packagedin either a cylindrical or prismatic housing.

[0009] In one embodiment of the electrochemical cell, the electrodes areprovided as two anode assemblies and one cathode assembly configuredsuch that each anode is positioned on either side of the cathodeassembly, and extending in opposing directions. At the center mostportion of the assembly there is an overlap of anodes. This assembly isthen wound about the overlapping region in a bi-directional fashion. Theresultant assembly produces a wound cell stack configuration with auniform contact of anode and cathode, such that the cell is balancedelectrochemically and provides for optimum volume utilization within thebattery enclosure. Each anode has one or more tabs that can be welded tothe case. Alternately, two cathode assemblies can be paired with oneanode assembly, with a resultant cathode tab welded to the case. In bothof the above configurations, the opposite electrode may contain one ormore tabs which are then electrically connected to the batteryfeedthrough pin.

[0010] An alternate embodiment of this invention provides for an anodeelectrode and a cathode electrode, wherein the electrodes are slotted.The electrodes are inserted, one into the other, essentially forming an“X”. Upon collapsing the electrodes, a variation of the above-describedinvention is obtained wherein the anode is approximately equallydisposed on opposite sides of the cathode, radiating outwardly from itsmidportion. This assembly is then wound from the center, resulting in apreferred cell stack assembly. This configuration provides for theadditional advantage of having the anode registered to the cathode, andmitigates the need for aligning two distinct anodes to the cathode.

[0011] Other features and advantages of the present invention willbecome apparent upon reading the following detailed description ofembodiments of the invention, when taken in conjunction with theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a side elevational view of the cathode strip andseparator of the present invention;

[0013]FIG. 2 is a side elevational view of the anode strip and separatorof the present invention;

[0014]FIG. 3 is a bottom plan view of the cell stack assembly of thepresent invention;

[0015]FIG. 4 is a side elevational view of the cell stack assembly ofthe present invention;

[0016]FIG. 5 is a partial plan view of the wound electrode assembly ofthe present invention;

[0017]FIG. 6 is a perspective view of an alternate embodiment of theelectrode strips of the present invention;

[0018]FIG. 7 is a partial plan view of the wound electrode assembly ofthe alternative embodiment; and

[0019]FIG. 8 is an exploded view of an electrochemical cell of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention is designed for high energy devices such asbatteries and capacitors and is adaptable in a wide variety of electrodeconfigurations and shapes for applications as capacitors and batteries,including aqueous and nonaqueous primary and secondary batteries.

[0021] Referring to FIG. 1, a first electrode 10 is preferably acontinuous structure comprising an active material 11 contacted to acurrent collector 12 (shown in dashed lines). The active material for acathode electrode is preferably comprised of a metal, a metal oxide, ametal sulfide, a mixed metal oxide, a carbonaceous material, or the likeand is combined with the current collector of a conductive material suchas a conductive screen. For an anode electrode, the preferred activematerial is an alkali metal selected from Group 1A of the Periodic Tableof Elements and contacted to an anode current collector. A preferredanode electrode comprises lithium contacted to a nickel currentcollector. In a preferred form of the present invention, the electrodestrip 10 is a cathode electrode having a set of cathode tabs 15 providedfor making an electrical connection to a positive terminal.

[0022] Turning to FIGS. 2 and 3, a second electrode 16 includes a pairof second electrode strips of a second electrode active material 17contacted to a current collector 18 (shown in dashed lines) disposed onopposite sides of the first electrode 10. The second electrode strips 16overlap along a midportion 19 of the first electrode 10 (FIG. 3).Preferably, the second electrode strips 16 are part of the anodeelectrode. The anode electrode strips 16 have anode tabs 22 that providefor electrical connection to a negative terminal.

[0023] As shown in FIGS. 1, 2 and 4, a separator material 13 is disposedbehind each electrode to prevent contact between overlayed layers ofelectrodes. Alternatively, the separator 13 is disposed in front of eachelectrode strip. In a preferred embodiment, which is not shown in thedrawings, a separator 13 in the form of an envelope encapsulates each ofthe first and second electrodes 10, 16. In that respect, whether theseparator 13 is disposed between immediately adjacent electrode stripsor, the separator serves as an envelope encapsulating at least one ofthe electrodes, the separator must prevent direct physical contactbetween the electrodes 10, 16.

[0024] Turning to FIG. 4, an electrode assembly according to the presentinvention comprises a cathode electrode 10 and two anode electrodes 16A,16B, which are each preferably elongate, flat, and rectangular. Theanode electrodes 16A, 16B are disposed on opposite sides of the cathode10 and aligned such that they overlap across the midportion 19 thereof.The anode electrodes 16A, 16B are a little more than half the length ofthe cathode electrode 10, and extend a short distance across themidportion 19 in order to overlap. Alternately, two cathode electrodeassemblies are paired with one anode electrode in a similar overlappingconfiguration.

[0025] From the alignment shown in FIGS. 3 and 4, the electrode strips10 and 16 are then folded about the overlapping region in abi-directional fashion to provide the electrode assembly 25. As shown inFIG. 5, those portions of anode strips 16A and 16B on the outside of theassembly 25 have the outside of the current collector devoid of anodeactive material. This is because there is no opposing cathode activematerial, and such anode active material would provide very little, ifany, additional volumetric efficiency. Also, the ends of the anodestrips 16A and 16B extend somewhat beyond the end of the cathodeelectrode 10 to fully utilize the discharge efficiency of the cathodeelectrode.

[0026] The term bi-directional refers to the fact that one side isfolded downwardly and the opposite side is folded upwardly, either insuccession or simultaneously, to generate the electrode assembly 25shown in FIG. 5. The electrode assembly 25 produces a wound cell stackconfiguration with uniform contact of anode and cathode electrodes suchthat the cell is balanced electrochemically and provides for optimumvolume utilization within the battery enclose.

[0027] An alternate embodiment of the present invention is shown inFIGS. 6 and 7. In this embodiment, a cathode electrode strip 50comprising a cathode active material 52 contacted to a cathode currentcollector 54 has a downwardly facing slot 53 disposed in a midportion 56thereof. An anode electrode strip 60 comprises an anode active material62 contacted to an anode current collector 64 and includes an upwardlyfacing slot 63 disposed in a midportion 66. The strips 50 and 60 aremoved together with the slots 53, 63 registering with each other to forma collapsible X-shaped assembly. In this embodiment, the anode strip 60extends outwardly a small distance past the opposed ends of the cathodestrip 50 and in a configuration such that the electrodes 50, 60 radiateoutwardly from the midportions 56, 66 of the other electrode. Theelectrode strips 50, 60 are then folded in a bi-directional fashion fromthe center or midportions 56, 66 to produce the wound electrode assembly75 shown in FIG. 7. The bi-directional folding is similar to thatdescribed with respect to the electrode assembly 25 shown in FIGS. 1 to5.

[0028] The completed electrode assembly 75 shown in FIG. 7 is similar tothe electrode assembly 25 in the respect that those portions of anodestrip 60 on the outside of the assembly have the outside of the currentcollector devoid of anode active material. As previously explained, thisis because there is no opposing cathode active material there, and suchanode active material would provide very little, if any, additionalvolumetric efficiency. Also, the ends of the anode strip 60 extendsomewhat beyond the respective ends of the cathode strip 50 to fullyutilize the discharge efficiency of the cathode electrode. Thisalternate embodiment provides the additional advantage of having theanode registered to the cathode and mitigates the need for aligning twodistinct anodes to the cathode.

[0029] The present electrode assemblies 25, 75 provide severaladvantages to cell design, including high energy density with lowinternal impedance. Additionally, the anode and cathode electrodes 10,16 for assembly 25 and the electrodes 50, 60 for assembly 75 arearranged in the cell in a way that provides efficient utilization of theactive components. The resultant wound cell stacks are configured suchthat they can be conveniently packaged in either a cylindrical orprismatic shaped casing. These casing shapes are well known to those ofordinary skill in the art. The electrode assemblies 25, 75 also providea cell stack construction in which the anode and cathode are uniformlyutilized during cell discharge. Finally, the assemblies 25, 75 provide acell having a relatively high inter electrode surface area which resultsin a high current rate capability. This is advantageous for use inapplications such as powering an implantable defibrillator.

[0030] A preferred primary electrode chemistry for the electrodeassemblies 25, 75 according to the present invention has the firstelectrode 10, 50 of a mixed metal oxide such as silver vanadium oxide(SVO), copper silver vanadium oxide (CSVO) or a fluorinated carbonaceousmaterial (CF_(x)), and the second electrode 16, 60 comprising lithium. ALi/SVO or Li/CSVO electrochemical couple is activated with anelectrolyte of 0.25M to 1.5M LiAsF₆ or LiPF₆ in a 50:50, by volume,mixture of propylene carbonate and 1,2-dimethoxyethane. For a Li/CF_(x)cell, the preferred electrolyte is 1.0M to 1.4M LiBF₄ inγ-butyrolactone. A preferred secondary chemistry has a carbonaceousnegative electrode and a lithiated counter electrode. A preferredlithiated material is lithium cobalt oxide. This couple is activatedwith an electrolyte of 1M LiPF₆ or 1M LiAsF₆ in ethylenecarbonate/1,2-dimethoxyethane (3:7).

[0031] Referring to FIGS. 1, 2 and 8, the anode tabs 22 can be welded tothe case 80 (negative). Alternately, two cathode assemblies can bepaired with one anode assembly with the resultant cathode tabs (notshown) welded to the case 80 (positive). In both of the aboveconfigurations, the opposite electrode may contain one or more tabs(cathode tabs 15) that are electrically connected to the batteryfeedthrough or terminal pin 82. The terminal pin 82 is electricallyinsulated from the lid 84 of the casing 80 by a glass-to-metal seal 86.Similar electrical connections for the cathode strip 50 and the anodestrip 60 are made for the electrode assembly 75 shown in FIGS. 6 and 7.

[0032] While the invention has been described in connection with certainpreferred embodiments, it is not intended to limit the scope of theinvention to the particular forms set forth, but, on the contrary, it isintended to cover such alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention, as definedby the appended claims.

What is claimed is:
 1. A method for providing an electrode assembly,comprising the steps of: (a) providing a first electrode strip having amidportion and a predetermined first length; (b) providing at least twosecond electrode strips having a second length shorter than the firstlength; (c) providing a separator material; (d) placing the at least twosecond electrode strips on opposite sides of the first electrode stripsuch that the at least two second electrode strips overlap along themidportion of the first electrode strip with the separator materialdisposed between the overlapping electrode strips; and (e)bi-directionally folding the first electrode strip and the at least twosecond electrode strips about the midportion of the first electrodestrip thereby forming a wound cell stack.
 2. The method of claim 1including providing the electrode assembly of either a primary or asecondary chemistry.
 3. The method of claim 1 including providing thefirst electrode strip comprising an anode electrode and providing the atleast two electrode strips comprising a cathode electrode.
 4. The methodof claim 1 including providing the first electrode strip comprising acathode electrode and providing the at least two second electrode stripscomprising an anode electrode.
 5. The method of claim 1 includingproviding the first electrode strip of a first electrode active materialand selecting the first electrode active material from the groupconsisting of SVO, CSVO and CF_(x), and providing the at least twosecond electrode strips of a second electrode active material comprisinglithium.
 6. The method of claim 1 including providing the firstelectrode strip of a first electrode active material comprising lithiumcobalt oxide, and the at least two second electrode strips of a secondelectrode active material comprising a carbonaceous material.
 7. Amethod for providing a electrochemical cell, comprising the step of: (a)providing a cathode strip comprising a cathode active material contactedto a cathode current collector and having a midportion and apredetermined first length; (b) providing at least two anode stripscomprising an active material contacted to an anode current collector,and each having a second length shorter than the first length; (c)positioning a separator material between the cathode strip and the atleast two anode strips; (d) positioning the at least two anode strips onopposite sides of the cathode strip in an overlapping fashion along themidportion of the cathode strip; (e) bi-directionally folding thecathode strip end and the anode strips about the midportion of thecathode strip to form a wound cell stack; and (f) activating the woundcell stack with an electrolyte.
 8. The method of claim 7 includingproviding the at least two anode strips each having at least one tabconnected to a battery case.
 9. The method of claim 7 includingproviding the cathode strip having a tab connected to a terminal pin.10. The method of claim 7 including providing the cell of either aprimary or a secondary chemistry.
 11. The method of claim 7 includingselecting the cathode active material of the cathode strip from thegroup consisting of SVO, CSVO and CF_(x), and providing the anode activematerial of the at least two anode strips comprising lithium.
 12. Themethod of claim 7 including providing the cathode active material of thecathode strip comprising lithium cobalt oxide, and providing the anodeactive material of the at least two anode strips comprising acarbonaceous material.
 13. The method of claim 7 including providing thecathode strip as a unitary member.
 14. The method of claim 7 includingproviding the at least two anode strips as unitary members.